A farmer had a watermelon patch. He discovered that some of the local kids had been helping themselves to a feast. He lost fruit whenever the melons were ripe and ready.
It was impractical to keep a constant watch over the watermelons, so he came up with an ingenious scheme: he placed a sign in the patch. It read: Warning – one of these watermelons contains cyanide.
The following day saw no fruits taken. The farmer was mighty pleased with his idea, until he noticed something written on his sign: Now you have two of them.
Thanks to Kaleidoscope for this one.